Image sensors for scanning document images, such as charge coupled devices (CCDs), typically have a row or linear array of photosites together with suitable supporting circuitry integrated onto a semiconductor chip. Usually, a sensor is used to scan line by line across the width of a document with the document being moved or stepped lengthwise in synchronism therewith. A typical architecture for such a sensor array is given, for example, in U.S. Pat. No. 5,153,421.
In a full-page-width image scanner, there is provided a linear array of photosensors which extends the full width of an original document, such as eleven inches. When the original document moves past the linear array, each of the photosensors converts reflected light from the original image into electrical signals. The motion of the original image perpendicular to the linear array causes a sequence of signals to be output from each photosensor, which can be converted into digital data.
A currently-preferred design for creating such a long linear array of photosensors is to provide a set of relatively small semiconductor chips, each semiconductor chip defining thereon a linear array of photosensors along with ancillary circuit devices. These chips are assembled end-to-end to form a single linear array of photosensors as disclosed in U.S. Pat. No. 5,473,513. However, a single chip having a linear array may also be used for sensing images and converting those images into electrical signals to be output from each photosensor, which can be converted into a digital signal. These chips can also be assembled end to end to form a full width array comprising a multiple parallel linear arrays of photosites.
With the gradual introduction of color-capable products into the office equipment market, it has become desirable to provide scanning systems which are capable of converting light from full-color images into separate trains of image signals, each train representing one primary color. In order to obtain the separate signals relating to color separations in a full-color image, one technique is to provide on a semiconductor chip multiple parallel linear arrays of photosensors, each of the parallel arrays being sensitive to one primary color. Typically, this arrangement can be achieved by providing multiple linear arrays of photosensors which are physically identical except for a translucent primary-color overlay over the photosensitive areas, or "photosites," for that linear array. In other words, the linear array which is supposed to be sensitive to red light only will have a translucent red layer placed on the photosites thereof, and such would be the case for a blue-sensitive array and a green-sensitive array. As the chip is exposed to an original full-color image, only those portions of the image which correspond to particular primary colors will reach those photosensors assigned to the primary color.
The most common substances for providing these translucent filter layers over the photosites is polyimide or acrylic. For example, polyimide is typically applied in liquid form to a batch of photosensor chips while the chips are still in undiced, wafer form. After the polyimide liquid is applied to the wafer, the wafer is centrifuged to provide an even layer of a particular polyimide. In order to obtain the polyimide having the desired primary-color-filtering properties, it is well known to dope the polyimide with either a pigment or dye of the desired color, and these dopants are readily commercially available. When it is desired to place different kinds of color filters on a single chip, a typical technique is to first apply an even layer of polyimide over the entire main surface of the chip (while the chip is still part of the wafer) and then remove the unnecessary parts of the filter by photo-etching or another well known technique. Typically, all of the filter layer placed over the chip is removed except for those areas over the desired set of photosites. Acrylic is applied to the wafer in a similar manner.